将字符串日期转换为纪元时间的意外值？

Question

The below gives: 1475020875000 . When I convert this epoch back to a human readable timestamp, I get: Wed, 28 Sep 2016 00:01:15 GMT , which is different from the initial date?

String date = "2016-09-27 20:01:15.0";
DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
long epoch = df.parse(date).getTime();

System.out.println(epoch);

Answer 1

You should specify Timezone for both input and output. You can use "z" to instantiate SimpleDateFormat and setTimeZone before using format method:

package stackoverflow;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.TimeZone;

public class Programa {

    public static void main(String[] args) throws ParseException {
        String date = "2016-09-27 20:01:15 GMT";
        DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss z");
        long epoch = df.parse(date).getTime();

        System.out.println(epoch);

        Date d = new Date(epoch);
        df.setTimeZone(TimeZone.getTimeZone("GMT"));
        String out = df.format(d);
        System.out.println(out);
    }

}

For available Timezones, try TimeZone.getAvailableIDs()

Answer 2

Worked fine here:

package stackoverflow;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class Programa {

    public static void main(String[] args) throws ParseException {
        String date = "2016-09-27 20:01:15.0";
        DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        long epoch = df.parse(date).getTime();

        System.out.println(epoch);

        Date d = new Date(epoch);
        String out = df.format(d);
        System.out.println(out);
    }

}

Answer 3

You certainly forgot to consider the Timezone to use. If you do not define it, the default (from the JVM) is taken, and can for example differ depending on your server.

Answer 4

Try LocalDateTime

    String date = "2016-09-27 20:01:15";
    DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    long epoch = df.parse(date).getTime();
    System.out.println(epoch);
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss", Locale.getDefault());
    LocalDateTime ld = LocalDateTime.parse(date, formatter);
    long epoch2 = ld.atZone(ZoneId.systemDefault()).toInstant().toEpochMilli();
    System.out.println(epoch2);
    Instant in = Instant.ofEpochMilli(epoch2);
    LocalDateTime ldt = LocalDateTime.ofInstant(in, ZoneId.systemDefault());
    System.out.println(ldt);

output

1474986675000
1474986675000
2016-09-27T20:01:15